In hospitals, clinics and the like, it is standard practice to sterilize various products such as gowns, drapes, sheets, dressings, and other articles, prior to use by placing them in an autoclave where they are subjected to steam sterilization. This practice is necessary to avoid infection and prevent contamination from the use of such articles where the same are not in a sterile condition and it is particularly important where the articles have previously been used in the care of other patients. As there is no visual way of determining whether a particular article is sterile or not it has been the practice to use, with the article, when placed in the autoclave, a colour change indicator element which changes colour under the sterilizing conditions of the autoclave, thus indicating that the particular article or package has been passed through the sterilizing cycle. The indicator may be in the form of a ribbon or card to which a colour change ink has been applied.
It is generally the practice in sterilizing such articles to gather several articles together, bundling the same in a porous wrap, and then the package, held together by tying with string or by pressure-sensitive adhesive tape, is placed in an autoclave together with a sterilizing indicator element, the sterilization indicator element may be either inserted in or applied to the package. Where pressure-sensitive adhesive tapes are used for this purpose, it is convenient to have the colour change indicator on the tape backing. By observation of the visible back of the tape which holds the package together, one can readily determine by its colour change whether or not the same has been passed through the sterilization cycle. Pressure-sensitive adhesive tapes of this type are, for example, described in U.S Pat. No. 2,889,799.
Typical sterilization conditions comprise subjecting the articles to saturated steam at a temperature of between 132.degree. C. and 140.degree. C., usually between 134.degree. C. and 140.degree. C. for a period of not less than 3 minutes. Another steam sterilization cycle comprises subjecting articles to 121.degree. C. for not less than 15 minutes. In order to ensure correct sterilizing conditions, the steam must penetrate unhindered to the centre of the load. This can be achieved only if all of the air is first removed from the sterilizer vessel and its load and this is accomplished typically by a process of evacuation and steam flushing of the sterilizing vessel and its load.
Failure to remove all of the air or the subsequent leakage of air into an evacuated chamber, or the introduction of air or non-condensible gases in the steam supply, causes gas pockets to remain within the porous load, usually in the inner regions thereof. In this case the temperature within the load might be lower in some places than that required during all or part of the sterilization process.
A standard test for the efficiency of air removal and/or the inclusion of non-condensible gases in autoclaves is known as the Bowie/Dick towel test. This test utilises a stack of standard linen Huckaback towels measuring some 270 mm high and about 300 mm .times.200 mm in plan. At about the vertical centre of the stack there is placed a sheet of paper bearing so-called autoclave tape or other indicator which undergoes a change in appearance in the presence of certain levels of moisture and temperature. The test pack is processed in the autoclave and the satisfactory result would show an even change in appearance across the whole of the indicator sheet, whereas the presence of air or non-condensible gas in the stack is indicated by a failure of the indicator to change its appearance in certain areas, usually at the centre. This test must be carried out daily before the autoclave is used for the production of sterile porous loads.
There are certain disadvantages associated with this known test. For example, the standard towel stack is too large to fit into many small-chambered autoclaves. Furthermore, the Huckaback towels are expensive and usually require to be laundered at least once a week. The towels require to be aired carefully between tests and they deteriorate gradually and become unusable within about 12 months of normal use. This is because the fibrous consistency becomes matted so that steam cannot readily diffuse through it, thus increasing the risk of misleading test results. Furthermore, Huckaback towels, being made from natural fibre, can give rise to exothermic reactions, i.e. they will absorb moisture very readily with the release of heat, when they are too dry. In the case of the Bowie/Dick test such a phenomenon can result in the temperature within the pack being higher than the temperature in the autoclave chamber and again this can lead to misleading results.
British Patent Specification No. 2143322A discloses a test kit for detecting the presence of air in a steam sterilizer which comprises a first porous mass of at least substantially man-made material, a second porous mass of a similar material, an indicator adapted to undergo a visual change under moist heat sterilising conditions and sandwiched between the masses thus to be in intimate contact therewith, and means for removably holding the masses and indicator in close superimposed relationship, said means being permeable to allow the free passage of air and steam to the external surfaces of said masses.
The test kit is generally in the form of a cuboid having a side dimension in the region of 12 to 15 cm and the indicator may conveniently comprise an autoclave tape or printed sheet having indicator in the shape of a St. Andrews cross or other pattern extending from side to side.
Experimental work has demonstrated that such a device as described is capable of detecting the presence of air in an autoclave. It is also clear from temperature measurements made within the device that the presence of air leads to a lower temperature at the centre of the test pack similar to that observed with the Huckaback towel pack used in the conventional Bowie/Dick test. The presence of air in the autoclave results in uneven change in the indicator, often incomplete colour change towards the centre of the device.
One of the most commonly used colour change media for use as an indicator contains sulfur and lead oxide. The sulfur in the presence of the lead oxide, under the conditions of the steam sterilization, changes from a yellow to a black colour as it is converted to lead sulfide. It has been the practice to mix the sulfur and the lead oxide together in equivalent molar proportions in an ink base which is then applied to an indicator card or to a back of a pressure-sensitive adhesive tape, which, in turn, is then associated with the articles to be sterilized. If the autoclave or other steam sterilizing equipment used is not functioning, or if the package to be sterilized inadvertently gets omitted from being placed in the autoclave, this can readily be determined by the fact that no reaction has occurred between the sulfur and the lead oxide as shown by the lack of colour change in the indicator. Similarly if air or non-condensible qas is present in the autoclave an incomplete reaction will occur in some areas resulting in incomplete generation of the black colour.
One difficulty with such inks is the tendency to stain when in contact with many articles. Staining can be the result of the transfer and reaction of sulfur as the formation of soluble materials during the colour change reaction staining causes unsightly marks on sheets and other articles which may prevent their use. Staining can be reduced by the provision of an overcoat over the printed ink requiring an extra processing step in the production of indicator elements thereby increasing the cost.
Another difficulty with the conventional sulfur lead oxide colour change inks is that they are relatively unstable, necessitating the practice of forming separate solutions of the sulfur and the lead oxide and then combining the two together just prior to using the ink for marking. This is apparently due to the fact that the sulfur and lead oxide tend to react while in the solvent medium used in forming the ink. If the ink solution is prepared several days prior to its use, a substantial reduction in colour change sensitivity results.
Where the colour change indicator is to be used in combination with a pressure-sensitive adhesive tape, as by printing the ink on the back of the tape, the inks containing the sulfur have the further disadvantage that the sulfur in the marking tends to migrate into the pressure-sensitive adhesive where the tape is wound on itself in roll form, thus further reducing the sensitivity of the marking to colour change, when the tape, which the ink marking thereon, is later exposed to steam sterilization conditions.
Although giving a good colour change from a light yellow to a deep black with fresh inks, the colour change indicator markings made from the active ingredients, lead oxide and sulfur, tend to lose much of their sensitivity over extended periods of storage. This is believed to be due to the oxidation of sulfur and is particularly prevalent under relatively hot conditions. Markings may not turn to the same deep black on being subjected to sterilizing conditions but tend to assume a dark grey appearance, with the result that the colour change is not nearly as marked as with fresh inks. This is particularly disadvantageous when the indicators are being used in testing the performance of an autoclave utilising the Bowie/Dick or similar test as misleading results may be obtained.
Despite these inherent disadvantages in colour change media using sulfur and lead oxide, such markings have continued to be widely used as steam sterilization indicators.
British Patent Specification No. 1211603 discloses an indicator material for determining whether steam sterilization has been applied comprising a mixture of lead carbonate and calcium sulfide. In the presence of steam at a temperature of about 120.degree. C. the calcium sulfide decomposes to form calcium hydroxide and hydrogen sulfide. The hydrogen sulfide in turn attacks lead carbonate to form black lead sulfide and carbonic acid.
Other known colour change indicators for indicating steam sterilization include e.g. the use of copper sulfite disclosed in U.S. Pat. No. 3 360 339, which provides a colour change from red to green under steam sterilization conditions, and have not found commercial favour.
It has now been found that there are a number of sulfur-containing compounds which will decompose to yield a sulfide under steam sterilization conditions with a marked colour change.